There is an FAA (Federal Aviation Administration) regulation regarding the design strength of landing gear for transport airplanes, which requires that the landing gear drag load be designed such as "A drag reaction equal to the vertical reaction multiplied by a coefficient of friction of 0.8, must be combined with the vertical ground reaction and applied at the ground contact point." This means that the entire landing gear assembly, including its structural connection to the airplane, must be designed for a braking drag load force based on eight tenths of the vertical weight down on the landing gear.
There is also an FAA regulation that states "A drag reaction lower than that prescribed--may be used if it is substantiated that an effective drag force of 0.8 times the vertical reaction cannot be attained under any likely loading condition." Therefore, a substantial weight savings can be realized if the landing gear assembly and its structural support can be designed to a ground coefficient of 0.5 to 0.6, which is still above the average actual of about 0.4. However, to comply with the FAA regulations it is required to prove that the 0.8 friction coefficient will not occur; and this can't really be proven for all conditions. So, another position would be to design a brake controller such that the braking torque will not generate a ground coefficient of friction greater than the supporting structure is designed for.
It is typical in the operation of airplane brakes, that they will have a high braking torque when they are cold and as the brakes warm up, the torque decreases; and as the rotation of the wheels ceases, the torque again increases. Also, the design engineer is faced with the problem of providing an airplane braking system that will produce enough torque for the situation of a long hot roll RTO (Refused Take Off) and yet not exceed the torque that generally results from rapid application of a cold brake in an emergency stopping situation where it is necessary that the brake produce the torque required for the minimum stopping distance and yet not exceed the torque that the landing gear assembly is designed for.
Further, some of the recent developments in lining materials and heat sinks have an even worse variation between running torque and peak torque than current conventional steel brakes e.g., structural carbon brakes have a ratio of heat absorption-to-weight, that is much better than conventional steel disk brake types. For an aircraft the size of a Boeing 747, there could be about a 1,600 lbs. weight savings. However, the trouble with carbon brakes is that when they are wet, they have a relatively low torque in comparison to the torque when dry. In order to compensate for this wide variation in torque between wet and dry operation, the brake pistons have to be built large enough to produce adequate torque when the brakes are wet. However, when the brakes are then operated in the dry condition, the torque produced will exceed the design limit and it becomes necessary to put more weight back into the landing gear assembly and airplane structural support to provide the necessary strength for reacting the high torque load; therefore, the advantages of using carbon brakes is compromised.